Memory devices, such as dynamic random access memory (DRAM) devices, operate in a wide range of operating voltages. Even with a regulated voltage supply, the range of the operating voltages can still be quite large due to the normal fluctuation of the regulated supply voltage. The regulated supply voltage level can also change because of the constantly changing current demand of a DRAM device.
In prior art FIG. 1, a known output driver circuit 10 includes a pull up transistor 11 and a pull down transistor 12 which are connected in series between a supply voltage rail VCCX and ground. The pull up transistor 11 is enabled in response to a logic high level at pull up node 14 to conduct and pull an output node 16 towards the supply voltage VCCX. Similarly, the pull down transistor 12 is enabled in response to a logic high level at a pull down node 18 to conduct and pull the output node 16 toward ground.
The speed at which the pull down transistor 12 pulls down the output node 16 depends upon the power supply voltage. Thus, in conventional output driver circuits, fluctuations in the supply voltage generally result in inconsistent drive which leads to inconsistent speed at which the pull down transistors pull the output down. The lower the supply voltage, the lower the drive, and consequently, the lower the switching speed for the pull down transistors of the output driver circuit.
While one could increase the drive at all supply levels, it would result in significant undesirable ground bounce at high supply levels.
There is a need in the art for an output driver circuit that compensates for variations in supply voltage. There is a further need for an output driver having less speed dependency on the supply voltage.